Bronchoalveolar lavage at fibreoptic bronchoscopy enables the peripheral bronchoalveolar free cell population, which consists mainly of lymphocytes and macrophages, to be sampled. The yield is sufficient for detailed morphological and functional investigation of these cells, which can be separated by exploiting the glass-adherent or phagocytic properties of macrophages. Such studies are of particular interest in patients with recurrent chest infections, to detect abnormalities of local cellular immunity; in pulmonary fibrosis to assess activity of the disease and effect of treatment by observing morphology and testing function of free lung cells; and in bronchial carcinoma to investigate local immune responses to the tumour. The procedure is held to be ethical in these circumstances. The lavage consists of irrigation of a segmental bronchus through the fibreoptic bronchoscope with up to 500 ml pH-corrected normal saline solution. It is contraindicated in those with respiratory or cardiac risk, but is tolerated well by most patients. Complications include acute respiratory distress, vasovagal syncope and fever with pulmonary infiltrates. Lavage is associated with a mean fall of PaO2 of 3.0 kPa (22.7 mmHg) and routine oxygen supplementation is recommended.